Combustion gas turbines are well known in the art. In general, those turbines include a compression section for compressing air entering the turbine, a combustion section following the compression section where the compressed air is combusted with fuel, and an expansion section, following the combustion section, where the combustion mixture from the combustion section is expanded to generate shaft work. The shaft work is transferred to an outside user that utilizes such shaft work. In many applications, the shaft work is transferred to an electrical generator that transforms the shaft work to electricity. The hot exhaust from the expansion section flows to a waste heat recovery unit where heat is recovered by generating steam or by providing heat to other media or heat utilizers.
Combustion gas turbines are constructed as single, double or triple shaft turbines Single shaft turbines include only one shaft utilized by both compression and expansion section at the same speed. A double shaft turbine includes two shafts, one shaft transferring work from the expansion section to the compression section and another shaft transferring work from the expansion section to a driven load. A triple shaft turbine includes one shaft transferring work from the expansion section to a portion of the compression section, a second shaft transferring work from the expansion section to another portion of the compression section, and a third shaft transferring work from the expansion section to the driven load. Although, single shaft turbines were used more often to generate work in the past, the use of double and triple shaft turbines has recently increased.
Several factors affect the performance and the work output generated by combustion gas turbines. One major factor is the inlet temperature of the air entering the compression stage of the turbine. Its effect on the power output of the gas turbine depends on the number of shafts of said turbine. In single shaft turbines, the output increases in a substantially linear fashion until it reaches a plateau as the inlet air temperature decreases. This correlation results from the fact that as the inlet temperature decreases, the density of the air increases whereby a larger mass of air flows through the turbine to generate an increased amount of work. FIG. 1 shows the above correlation of electricity generated versus inlet temperature of air for a single shaft combustion gas turbine operating with natural gas fuel at sea level, sixty (60) percent relative humidity, 60 hz, inlet loss of 4 inches of H.sub.2 O, exhaust loss of 10 inches of H.sub.2 O, and with no steam or water injection for control of nitrogen oxides emissions. The abscissa shows the inlet temperature of the combustion air in degrees Fahrenheit (.degree.F.) and the ordinate shows the output at the generator terminals in kilowatts (kw).
In multi-shaft, i.e., double or triple shaft gas turbines, the correlation between output and inlet air temperature is different in that, although the output increases as the air inlet temperature decreases in a particular temperature range, the output reaches a maximum at the lowest point of that range and decreases as the temperature decreases below that point. Referring now to FIG. 2, there is shown a graph depicting the correlation between electrical output versus air inlet temperature of a double shaft General Electric LM2500 gas turbine generating electricity and operating with natural gas fuel at sea level, sixty (60) percent relative humidity, 60 hz, inlet loss of 4 inches of H.sub.2 O, exhaust loss of 10 inches of H.sub.2 O, and with water injection for control of nitrogen oxides emissions, the amount of the water being sufficient to meet the typical regulatory emission requirements of nitrogen oxides of about 42 parts per million on a dry basis. The abscissa shows the inlet temperature of the combustion air in degrees Fahrenheit (.degree.F.), and the ordinate shows the output at the generator terminals in kilowatts (kw). FIG. 2 shows that the electrical output increases from about 18,500 kilowatts to about 24,300 kilowatts as the inlet temperature of the air decreases from 100.degree. F. to 35.degree. F. As the temperature decreases below 35.degree. F., the electrical output decreases with such temperature decrease. Therefore, it appears from FIG. 2 that the most desirable air inlet temperature for that particular turbine is about 35.degree. F.
Triple shaft gas turbines have a similar maximum electrical output achieved at a particular air inlet temperature. Referring now to FIG. 3, there is shown a graph depicting the correlation between electrical output and inlet temperature of air in a triple shaft General Electric LM5000 gas turbine generating electricity and operating with natural gas fuel at sea level, sixty (60) percent relative humidity, 60 hz, inlet loss of 4 inches of H.sub.2 O, exhaust loss of 10 inches of H.sub.2 O, with steam injection for control of oxides of nitrogen emissions (about 42 parts per million on a dry basis), and additional steam injection for power augmentation. The abscissa shows the inlet temperature of the combustion air in degrees Fahrenheit (.degree.F.), and the ordinate shows the output at the generator terminals in kilowatts (kw). There is shown that the electrical output increases from about 39,500 kilowatts to about 53,000 kilowatts as the temperature decreases from 100.degree. F. to 40.degree. F. The electrical output starts decreasing beyond that point (40.degree. F.) as the inlet temperature of the air decreases. Therefore, it is apparent that it is desirable to operate the gas turbine with an air inlet temperature of about 40.degree. F.
In the past, because gas turbines have been more commonly used to generate power in hot climates, only coolers have been used to decrease the inlet temperature of the air to increase the power output. Heaters have not been used to increase the air inlet temperature towards the optimum air inlet temperature, as demonstrated by the above graphs, to increase the power output towards its maximum. As a result, the multishaft gas turbines previously used in cold environments did not produce the maximum output achievable by those turbines.
According to the present invention, a method and an apparatus are disclosed to increase the inlet temperature of the air in cold climates to obtain the optimum air inlet temperature by heating the air in a heater. The heater may be the same apparatus that is used to cool the air to reach the optimum air inlet temperature when the ambient temperature is high due to hot weather conditions. In those instances, the apparatus is sometimes referred to herein as the heater/cooler. The heater may also be a separate apparatus which is operated only during the cold periods while a separate cooling apparatus is used alone during the hot periods.
Another problem encountered in the past in cold climates has been the formation of ice at the inlet of the gas turbine caused by the condensation of water thereon. The accumulation of such ice is oftentimes very rapid and causes plugging of the filter surface, possible engine damage from ice formed at the engine bellmouth, and a total shutdown of the gas turbine. In the past, this problem has been addressed by flowing hot exhaust gases from the outlet of the turbine through a heat exchanger and over the inlet thereof to prevent such icing. One disadvantage of that method was that it required the addition of special equipment such as jackets around the inlet. Another disadvantage was that the hot gases were available at substantially high temperatures whereby they formed hot spots around the inlet of the turbine. Still another disadvantage was that the temperature at the inlet of the gas turbine could not be easily controlled. The addition of the heater disclosed by the present invention prevents the formation of ice at the inlet of the gas turbine while eliminating the problems of previous deicing techniques.
These and other advantages of the present invention will become apparent from the following description and drawings.